Sunday, April 15, 2012

Addition and Condensation Polymerization

Addition polymers: They are formed by the addition of olifinic monomers without the elimination of byproducts.  Polymerization is brought about by linking together simple unsaturated molecules such as alkenes (ethylene and propylene) or substituted alkenes (vinyl; CH2=CH2-group).  Examples: polythene, polypropylene, PVC, Plexiglas, etc.

Polyethylene is made by the addition polymerization of ethylene. Addition Polymerization is initiated using small amounts of substances called initiators. Water soluble potassium persulphate and dibenzoyl peroxide which is soluble in organic solvents have been used as initiators. The introduction of Ziegler catalysts as initiators has made possible the synthesis of polymers which had earlier been found to be either difficult or impossible. One of the Ziegler –Natta catalysts is a combination of titanium tetra chloride and triethyl aluminium. A characteristic feature of the catalyst is that it imparts a regular structure to the polymer.